In an optical-disc of the type presently used with a DAD (Digital Audio Disc) player or a video disc player, audio or video information is recorded in optically coded form as a series of recesses or pits formed in the information carrying face of the optical-disc and arranged in either a single spiral track or a plurality of concentric tracks about the center axis of the optical-disc. The audio or video information thus recorded is reproduced by optically scanning the individual recesses or pits along the spiral track or the concentric tracks by means of a convergent spot developed from a laser light beam which is utilized to detect the lengths and spacings between the scanned recesses or pits. During reproduction, the optical-disc is usually rotated about the center axis thereof at a constant linear velocity (CLV) or a constant angular velocity (CAV), and the convergent spot of the laser light beam is displaced radially relative to the optical-disc by a tracking device or a pick-up unit which is a part of the DAD or video disc player. The laser beam is directed to a selected track location and reflected by the information carrying face of the optical-disc or is passed through the optical-disc. The beam of light thus reflected by or passed through the optical-disc is then converted into an electric signal by means of a photoelectric transducer mounted on the pick-up unit so as to facilitate further conversion into an audio or video signal.
In order to reproduce the information recorded on the optical-disc, the convergent spot of the laser light beam must be precisely applied to the selected track location on the optical-disc. To this end, it is known to employ a method in optical-disc information reproducing apparatus wherein a light beam, exclusively used for scanning, is generated separately from the light beam used for reproducing the audio or video information signal, with a photoelectric transducer being used for detecting the scanning light beam and effect scanning control. Another method employed involves extracting the audio or video information signal and the scanning signal all with a single light beam.
According to the latter method, the photoelectric transducer includes a plurality of photodetecting units, for example, four units. An image of the pit reflected by the single light beam is converted into respective electric signals by the four photodetecting units. The signals from the four photodetecting units are then converted into signals carrying audio or video information, focusing error information and tracking error information. The scanning error information, that is, the focusing error information signal and tracking error information signal thus detected, are introduced into a focusing actuator and a tracking actuator for driving an objective lens mounted on the pick-up unit vertically and radially relative to the optical-disc so as to direct the convergent spot to the selected track location accurately. A focusing actuator and a tracking actuator together comprise a scanning servo for the convergent spot of the laser light beam.
Generally, the focusing actuator and the tracking actuator must have relatively high response characteristics to make the convergent spot strictly follow quick fluctuations of the selected track location typical during reproduction conditions. However, when dropouts occur in the focusing error information signal or the tracking error information signal due to dust or scars on the optical-disc, the focusing actuator and the tracking actuator mistakingly cause the convergent spot to jump from the correct track location. Because the servo system is apt to become out of control when these signals are in error for more than a certain time, the allowable time for signal dropout varies according to the response characteristics of the servo system. That is, the allowable dropout time is shorter in a servo system with high response characteristics, and is longer in a servo system with low response characteristics. By lowering the response characteristics, malfunctioning of the scanning servo can be prevented. Conventional focusing and tracking actuator systems have therefore been designed which vary their response characteristics upon detection of information dropout.
Scanning servo systems employed to date have several drawbacks, however. The convergent spot is still apt to jump from the selected track location because the focusing actuator and the tracking actuator fail to vary their response characteristics during the short period at the leading portion of each information dropout. If this drawback is prevented by downgrading the higher response characteristics of the focusing actuator and the tracking actuator, the conventional scanning servo system has another drawback in that the focusing actuator and the tracking actuator then fail to make the convergent spot follow quick fluctuations of the selected track location caused by, for example, a mechanical shock to the DAD or video disc player.